1. Field of the Invention
This invention relates to the use of elements with positive temperature coefficient resistivity (PTC elements) in circuit breakers.
2. Description of the Related Art
A circuit breaker protects circuits not only in short circuit situations but also in overload situations. For instance, according to UL489 requirements, the circuit breaker must trip within an hour when current reaches 135% of the ampere rating of the breaker. Typically, a bimetal is used in existing circuit breakers for overload protection. The bimetal is a current carrying part in low ampere rated circuit breakers. However, in high ampere rated circuit breakers, the bimetal does not carry current directly. When an overload situation occurs, the high current increases the temperature of the bimetal and the bimetal is deflected by the heat, causing the circuit breaker to trip.
Circuit breakers including bimetal must be calibrated which significantly raises the cost of manufacturing and include many other disadvantages related to using the bimetal and calibration. However, even with calibration the bimetal does not always behave consistently and a calibrated circuit breaker will not always trip at the set overload rating.
The method and apparatus of the present invention uses one or more conductive polymer elements such as a positive temperature coefficient resistivity element (PTC element) to replace the bimetal in a circuit breaker thereby eliminating the need for calibration as well as other problems associated with the use of bimetal in circuit breakers.
An important issue in practically designing a circuit breaker with the PTC element replacing the bimetal is the tolerance level of the PTC element. According to the published specifications of typical PTC elements, such as those manufactured by Raychem or Bourns, the PTC element's tolerance is usually too large to be practically used in circuit breaker design. For an example, Raychem's RUE900.TM. has a hold current of 9 A and a trip current of 18 A at 20.degree. C. The hold current is defined as the maximum current at which the RUE900.TM. will always trip the circuit breaker at 20.degree. C. In other words, the PTC element is only guaranteed to trip when the current reaches 200% of the ampere rating at room temperature. As mentioned before, circuit breakers are required by the UL489 standard to trip if current reaches 135% of the ampere rating.
The present invention discloses a method for narrowing the tolerance of the PTC element providing for circuit breakers practically designed with the PTC element replacing bimetal.